The rate of entry to slow inactivation was not affected by coexpression of WT β1, but was slightly prolonged by coexpression of β1/C121W. Entry to slow inactivation at 0 mV was measured using a sequential entry protocol (inset). Expression of wild-type β1 (α + β1, ■) or α and β1 C-terminal deletion mutant (α + β1/CTdel, ▲) did not affect the rate of entry to slow inactivation, as compared to coexpression of α subunit alone (α, ○). The β1 N-terminal mutation C121W (α + β1/C121W, ∇) prolonged the rate of entry to slow inactivation by 1.4-fold (p < 0.05).